Compounds that selectively control chemical synaptic transmission offer therapeutic utility in treating disorders that are associated with dysfunctions in synaptic transmission. This utility may arise from controlling either pre-synaptic or post-synaptic chemical transmission. The control of synaptic chemical transmission is, in turn, a direct result of a modulation of the excitability of the synaptic membrane. Presynaptic control of membrane excitability results from the direct effect an active compound has upon the organelles and enzymes present in the nerve terminal for synthesizing, storing, and releasing the neurotransmitter, as well as the process for active re-uptake. Postsynaptic control of membrane excitability results from the influence an active compound has upon the cytoplasmic organelles that respond to neurotransmitter action.
An explanation of the processes involved in chemical synaptic transmission will help to illustrate more fully the potential applications of the invention. (For example, a fuller explanation of chemical synaptic transmission Hoffman et al., “Neuro-transmission: The autonomic and somatic motor nervous systems.” in Goodman and Gilman's. The Pharmacological Basis of Therapeutics. 9th ed., J. G. Hardman. L. E. Limbird, P. B. Molinoff. R. W. Ruddon, and A. Goodman Gilman, eds., Pergamon Press, New York. (1996), pp. 105-139).
Typically, chemical synaptic transmission begins with a stimulus that depolarizes the transmembrane potential of the synaptic junction above the threshold that elicits an all-or-none action potential in a nerve axon. The action potential propagates to the nerve terminal where ion fluxes activate a mobilization process leading to neurotransmitter secretion and “transmission” to the postsynaptic cell. Those cells which receive communication from the central and peripheral nervous systems in the form of neurotransmitters are referred to as “excitable cells.” Excitable cells are cells such as nerves, smooth muscle cells, cardiac cells and glands. The effect of a neurotransmitter upon an excitable cell may be to cause either an excitatory or an inhibitory postsynaptic potential (EPSP or IPSP, respectively) depending upon the nature of the postsynaptic receptor for the particular neurotransmitter and the extent to which other neurotransmitters are present. Whether a particular neurotransmitter causes excitation or inhibition depends principally on the ionic channels that are opened in the postsynaptic membrane (i.e., in the excitable cell).
EPSPs typically result from a local depolarization of the membrane due to a generalized increased permeability to cations (notably Na+ and K+), whereas IPSPs are the result of stabilization or hyperpolarization of the membrane excitability due to a increase in permeability to primarily smaller ions (including K+ and Cl−). For example, the neurotransmitter acetylcholine excites at skeletal muscle junctions by opening permeability channels for Na+ and K+. At other synapses, such as cardiac cells, acetylcholine can be inhibitory, primarily resulting from an increase in K+ conductance.
The biological effects of the compounds of the present invention result from modulation of a particular subtype of acetylcholine receptor. It is, therefore, important to understand the differences between two receptor subtypes. The two distinct subfamilies of acetylcholine receptors are defined as nicotinic acetylcholine receptors and muscarinic acetylcholine receptors. (See Goodman and Gilman's, The Pharmacological Basis of Therapeutics, op. cit.).
The responses of these receptor subtypes are mediated by two entirely different classes of second messenger systems. When the nicotinic acetylcholine receptor is activated, the response is an increased flux of specific extracellular ions (e.g. Na+, K+ and Ca++) through the neuronal membrane. In contrast, muscarinic acetylcholine receptor activation leads to changes in intracellular systems that contain complex molecules such as G-proteins and inositol phosphates. Thus, the biological consequences of nicotinic acetylcholine receptor activation are distinct from those of muscarinic receptor activation. In an analogous manner, inhibition of nicotinic acetylcholine receptors results in still other biological effects, which are distinct and different from those arising from muscarinic receptor inhibition
As indicated above, the two principal sites to which drug compounds that affect chemical synaptic transmission may be directed are the presynaptic membrane and the post-synaptic membrane. Actions of drugs directed to the presynaptic site may be mediated through presynaptic receptors that respond to the neurotransmitter which the same secreting structure has released (i.e., through an autoreceptor), or through a presynaptic receptor that responds to another neurotransmitter (i.e., through a heteroreceptor). Actions of drugs directed to the postsynaptic membrane mimic the action of the endogenous neurotransmitter or inhibit the interaction of the endogenous neurotransmitter with a postsynaptic receptor.
Classic examples of drugs that modulate postsynaptic membrane excitability are the neuromuscular blocking agents which interact with nicotinic acetylcholine-gated channel receptors on skeletal muscle, for example, competitive (stabilizing) agents, such as curare, or depolarizing agents, such as succinylcholine.
In the central nervous system (CNS), postsynaptic cells can have many neurotransmitters impinging upon them. This makes it difficult to know the precise net balance of chemical synaptic transmission required to control a given cell. Nonetheless, by designing compounds that selectively affect only one pre- or postsynaptic receptor, it is possible to modulate the net balance of all the other inputs. The more that is understood about chemical synaptic transmission in CNS disorders, the easier it would be to design drugs to treat such disorders.
Knowing how specific neurotransmitters act in the CNS allows one to predict the disorders that may be treatable with certain CNS active drugs. For example, dopamine is widely recognized as an important neurotransmitter in the central nervous systems in humans and animals. Many aspects of the pharmacology of dopamine have been reviewed by Roth and Elsworth, “Biochemical Pharmacology of Midbrain Dopamine Neurons” Psychopharmacology: The Fourth Generation of Progress, F. E. Bloom and D. J. Kupfer. Eds. Raven Press, NY, 1995, pp 227-243). Patients with Parkinson's disease have a primary loss of dopamine containing neurons of the nigrostriatal pathway, which results in profound loss of motor control. Therapeutic strategies to replace the dopamine deficiency with dopamine mimetics, as well as administering pharmacologic agents that modify dopamine release and other neurotransmitters have been found to have therapeutic benefit (“Parkinson's Disease”, Psychopharmacology: The Fourth Generation of Progress. op. cit., pp 1479-1484).
New and selective neurotransmitter controlling agents are still being sought, in the hope that one or more will be useful in important, but as yet poorly controlled, disease states or behavior models. For example, prior to the present invention dementia, such as is seen with Alzheimer's disease or Parkinsonism, remained largely untreatable. Symptoms of chronic alcoholism and nicotine withdrawal involve aspects of the central nervous system, as does the behavioral disorder Attention Deficit Disorder (ADD). Specific agents for treatment of these and related disorders are few in number or nonexistent.
A more complete discussion of the possible utility as CNS active agents of compounds with activity as cholinergic ligands selective for neuronal nicotinic receptors, (i.e., for controlling chemical synaptic transmission) may be found in U.S. Pat. No. 5,472,958 the disclosure of which is incorporated herein by reference.
Existing acetylcholine agonists are therapeutically suboptimal in treating the conditions discussed above. For example, such compounds have unfavorable pharmacokinetics (e.g., arecoline and nicotine), poor potency and lack of selectivity (e.g., nicotine), poor CNS penetration (e.g., carbachol) or poor oral bioavailability (e.g., nicotine). In addition, other agents have many unwanted central agonist actions, including hypothermia, hypolocomotion and tremor and peripheral side effects, including miosis, lachrymation, defecation and tachycardia (Benowitz et al., in Nicotine Psychopharmacology, S. Wonnacott, M. A. H. Russell, & I. P. Stolerman, eds., Oxford University Press. Oxford, 1990, pp. 112-157; and M. Davidson, et al., in Current Research in Alzheimer Therapy, E. Giacobini and R. Becker, ed.; Taylor & Francis: New York, 1988; pp 333-336).
The use of cholinergic channel modulators to treat Parkinson's and Alzheimer's Diseases is described by M. Williams et al. “Beyond the Tobacco Debate: Dissecting Out the Therapeutic Potential of Nicotine”, Exp. Opin. Invest. Drugs 5, pp. 1035-1045 (1996). Short-term improvement of nonsmoking patients suffering from depression by treatment with nicotine patches is described by R. J. Salin-Pascual et al., “Antidepressant Effect of Transdermal Nicotine Patches in Non-Smoking Patients with Major Depression”, J. Clin. Psychiatry, v. 57 pp. 387-389 (1996).
WO 94/08922 describes pyridyl ether compounds which enhance cognitive function. U.S. patent application Ser. Nos. 08/474,873 and 08/485,537 describe certain substituted pyridyl ether compounds as well as other compounds which also act at the nicotinic acetylcholine receptor to stimulate or inhibit neurotransmitter release. WO 96/31475 describes certain 3-substituted pyridine derivatives which are described as being useful for a variety of disorders as modulators of acetylcholine receptors. While some of these references have alluded to pain control as a potential use of the compounds or analogs recited therein, the Applicants have discovered that compounds of formula I shown below have a surprising and unexpected analgesic effect.
In addition, cholinergic channel modulators may be useful in treating pain. The search for more potent and more effective pain controllers or analgesics continues to be a significant research goal in the medical community. A substantial number of medical disorders and conditions produce pain as part of the disorder or condition. Relief of this pain is a major aspect of ameliorating or treating the overall disease or condition. Pain and the possible alleviation thereof is also attributable to the individual patient's mental condition and physical condition. One pain reliever, or a class, may not be effective for a particular patient, or group of patients, which leads to a need for finding additional compounds or pharmaceuticals which are effective analgesics. Opioid and non-opioid drugs are the two major classes of analgesics (Dray, A. and Urban, L., Ann. Rev. Pharmacol. Toxicol., 36: 253-280, 1996). Opioids, such as morphine, act at opioid receptors in the brain to block transmission of the pain signals in the brain and spinal cord (Cherney, N. I. Drug, 51:713-737, 1996). Opioids such as morphine have abuse and addiction liability. Non-opioids such as non-steroid anti-inflammatory agents (NSAIDs) typically, but not exclusively, block the production of prostaglandins to prevent sensitization of nerve endings that facilitate the pain signal to the brain (Dray, et al, Trends in Pharmacol. Sci., 15: 190-197, 1994; Carty, T. J. and Marfat. A., “COX-2 Inhibitors. Potential for reducing NSAID side-effects in treating inflammatory diseases”, Emerging Drugs: Prospect for Improved Medicines. (W. C. Bowman, J. D. Fitzgerald, and J. B. Taylor, eds.). Ashley Publications Ltd., London. Chap. 19., pp. 391411). Most of the commonly prescribed over-the-counter (OTC) NSAIDs are also commonly associated with at least one side effect or another, such as stomach ulceration or pain. For example, NSAIDs such as aspirin are also known to cause irritation and ulceration of the stomach and duodenum.
Certain compounds, with primary therapeutic indications other than analgesia, have been shown to be effective in some types of pain control. These are classified as analgesic adjuvants, and include tricyclic antidepressants (TCAs) and some anticonvulsants such as gabapentin (Williams et al., J. Med. Chem. (1999), 42, 1481-1500). The exact mechanism of action of these drugs is not fully understood, but they are used increasingly for treatment, especially for pain resulting from nerve injury due to trauma, radiation, or disease.
The compounds of the present invention are novel, have utility in treating pain and may also have utility in treating disorders and medical conditions listed herein. The compounds of the present invention may also have utility when administered in combination with an opioid such as morphine, a non-steroid anti-inflammatory agent such as aspirin, a tricyclic antidepressant, or an anticonvulsant such as gabapentin or pregabalin for treating disorders and medical conditions listed herein.